Plasma cutting uses a constricted electric arc to heat a gas flow to the plasma state. The energy from the high temperature plasma flow locally melts the workpiece. The energy from the high temperature plasma flow locally melts the workpiece. For many cutting processes, a secondary gas flow (also known as a shield gas flow, or shield flow) is used to protect the torch and assist the cutting process. The momentum of the high temperature plasma flow and the shield flow help remove the molten material, leaving a channel in the workpiece known as a cut kerf (“kerf”).
Relative motion between the plasma torch and the workpiece allows the process to be used to effectively cut the workpiece. The shield gas interacts with the plasma gas and the surface of the workpiece and plays a critical role in the cutting process. Downstream of the nozzle orifice, the plasma and shield gas flows come into contact enabling heat and mass transfer.
FIG. 1 is a diagram of a known automated plasma torch system. Automated torch system 10 can include a cutting table 22 and torch 24. An example of a torch that can be used in an automated system is the HPR260 auto gas system, manufactured by Hypertherm, Inc., of Hanover, N.H. The torch height controller 18 can be mounted to a gantry 26. The automated system 10 can also include a drive system 20. The torch is powered by a power supply 14. The plasma arc torch system can also include a gas console 16 that can be used to regulate/configure the gas composition (e.g., gas types for the shield gas and plasma gas) and the gas flow rates for the plasma arc torch. An automated torch system 10 can also include a computer numeric controller 12 (CNC), for example, a Hypertherm Automation Voyager, manufactured by Hypertherm, Inc., Hanover, N.H. The CNC 12 can include a display screen 13 which is used by the torch operator to input or read information that the CNC 12 uses to determine operating parameters. In some embodiments, operating parameters can include cut speed, torch height, and plasma and shield gas composition. The display screen 13 can also be used by the operator to manually input operating parameters. A torch 24 can also include a torch body (not shown) and torch consumables that are mounted to the front end of a torch body. Further discussion of CNC 12 configuration can be found in U.S. Patent Publication No. 2006/0108333, assigned to Hypertherm, Inc., the disclosure of which is incorporated herein by reference in its entirety.
FIG. 2 is a cross-sectional view of a known plasma arc torch tip configuration, including consumable parts and gas flows. The electrode 27, nozzle 28, and shield 29 are nested together such that the plasma gas 30 flows between the exterior of the electrode and the interior surface of the nozzle. A plasma chamber 32 is defined between the electrode 27 and nozzle 28. A plasma arc 31 is formed in the plasma chamber 32. The plasma arc 31 exits the torch tip through a plasma nozzle orifice 33 in the front end of the nozzle to cut the workpiece 37. The shield gas 34 flows between the exterior surface of the nozzle and the interior surface of the shield. The shield gas 34 exits the torch tip through the shield exit orifice 35 in the front end of the shield, and can be configured to surround the plasma arc. In some instances, the shield gas also exits the torch tip through bleed holes 36 disposed within the shield 29. A portion of the shield gas flow can enter the cut kerf with the plasma gas and form a boundary layer between the cutting arc and the workpiece surface 37. The composition of this boundary layer influences the heat transfer from the arc to the workpiece surface and the chemical reactions that occur at the workpiece surface. An example of plasma torch consumables are the consumable parts manufactured by Hypertherm, Inc., of Hanover, N.H. for HPR 130 systems, for cutting mild steel with a current of 80 amps. The nozzle 28 can be a vented nozzle, (e.g., comprising an inner and outer nozzle piece and a bypass channel formed between the inner and outer nozzle pieces directs the bypass flow to atmosphere), as described in U.S. Pat. No. 5,317,126 entitled “Nozzle And Method Of Operation For A Plasma Arc Torch” issued to Couch et al., which is owned by the assignee of the instant application and the disclosure of which is incorporated herein by reference in its entirety.
Internal features (e.g., hole features, substantially circular holes, slots, etc.) cut with plasma arc torches using known methods can result in defects, such as, for example, protrusions, divots, “bevel” or “taper.” Bevel or taper is where a feature size at a bottom side of the workpiece is smaller than the feature size at the top side of the plate. For example, the diameter of an internal feature (e.g., a hole/hole feature) at the top of the workpiece should be cut to match the size of a bolt to pass through the internal feature. If a hole feature has defects, such as, protrusions, divots, bevel or taper, the defects in the hole feature can cause the hole feature diameter to vary from the top of a workpiece to the bottom of the workpiece. Such defects can prevent the bolt from passing through the bottom of the workpiece. Secondary processes, such are reaming or drilling are required to enlarge the diameter of the bolt hole feature at the bottom of the workpiece. This prior method of ensuring hole cut quality can be time consuming, suggesting that a more efficient method of cutting holes and contours in a single workpiece is needed.
Numerous gas mixtures can be used for both plasma and shield gas in plasma cutting processes. For example, oxygen is used as the plasma gas and air as the shield gas for the processing of mild steel. Some low current processes (e.g., less than 65 A) use oxygen as both the plasma gas and shield gas to cut thin material (e.g., workpieces less than 10 gauge). The oxygen plasma gas/air shield gas combination is popular for mild steel at arc currents above 50 amps, due to the ability to produce large parts with good quality and minimal dross at high cutting speeds. Such cutting processes have certain drawbacks. For example, though the oxygen plasma gas/air shield gas configuration can cleanly cut large sections with straight edges (e.g., contours), such a gas combination is unable to cut high quality hole features. Instead, hole features cut with oxygen plasma gas and air shield gas has a substantial bevel or “taper”.
Traditionally, to correct defects in the hole feature, such as, for example, a “protrusion” (e.g., excess material) where the lead-in of a cut transitions into a perimeter of the cut, the arc is left on after cutting the perimeter to “clean up” the defect left by the lead-in by cutting the unwanted excess material. This process is called “over burn.” Over burning, however, can result in removing too much material, leaving an even larger defect (e.g., leaving a divot in place of the protrusion).